Method of making flexible magnetic printing plates

ABSTRACT

A flexible resilient printing plate adapted for use on a magnetic printing cylinder is formed by mixing a ferrous metal powder within a volume of liquid plastisol, casting the mixture onto a face layer of cured plastisol to form a base layer, and then heating the combined layers to cure the base layer and fuse it to the face layer to form a flexible multiple layer mold sheet with a substantially uniform thickness, and a porous fibrous impression mat is place in contact with the face layer. The mat and mold sheet are heated and are place within a diaphragm press with a porous cloth between the mold sheet and the diaphragm. The mold sheet is pressed into the mat to form a printing plate which is allowed to cool during the pressing operation. The mat and plate are magnetically retained on the bed of a shaver, and the base layer of the plate is shaved to produce a precise uniform caliper. After the printing plate is shaved and stripped from the porous mat, it is reheated until it relaxes. The reheated plate is place on a drum having an outer diameter the same as the magnetic cylinder and is held in firm contact with the drum until the plate cools. The plate is also produced without the face layer of plastisol so that the metal powder is dispersed throughout the entire plate.

H. L. BISHOP Sept. 24, 1974 METHOD OF MAKING FLEXIBLE MAGNETIC PRINTINGPLATES 3 Sheets-Sheet 1 Filed Sent. 21, 1972 p 4. 1974 H. L. BISHOP3,837,959

METHOD OF MAKING FLEXIBLE MAGNETIC PRINTING PLATES Filed Sent. 21, 19723 Sheets-Sheet 2 4 W--v- 11-41-, v

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METHOD OF MAKING FLEXIBLE MAGNETIC .PRINTING PLATES Filed Sept. 21, 1972FIG-2O 29a 294 I :11 280 L j j wam ggm wf f 300 3 Sheets-Sheet 3 UnitedStates Patent O 3,837,959 METHOD OF MAKING FLEXIBLE MAGNETIC PRINTINGPLATES Homer L. Bishop, P.O. Box 35, West Carrollton, Ohio 45449Continuation-impart of application Ser. No. 194,651, Nov.

1, 1971, now Defensive Publication No. T903,017. This application Sept.21, 1972, Ser. No. 290,852

Int. Cl. B29c 19/00; B41c 3/06 US. Cl. 156-246 7 Claims ABSTRACT OF THEDISCLOSURE A flexible resilient printing plate adapted for use on amagnetic printing cylinder is formed by mixing a ferrous metal powderwithin a volume of liquid plastisol, casting the mixture onto a facelayer of cured plastisol to form a base layer, and then heating thecombined layers to cure the base layer and fuse it to the face layer toform a flexible multiple layer mold sheet. The base layer is shaved toprovide the mold sheet with a substantially uniform thickness, and aporous fibrous impression mat is placed in contact with the face layer.The mat and mold sheet are heated and are placed within a diaphragmpress with a porous cloth between the mold sheet and the diaphragm. Themold sheet is pressed into the mat to form a printing plate which isallowed to cool during the pressing operation. The mat and plate aremagnetically retained on the bed of a shaver, and the base layer of theplate is shaved to produce a precise uniform caliper. After the printingplate is shaved and stripped from the porous mat, it is reheated untilit relaxes. The reheated plate is placed on a drum having an outerdiameter the same as the magnetic cylinder and is held in firm contactwith the drum until the plate cools. The plate is also produced withoutthe face layer of plastisol so that the metal powder is dispersedthroughout the entire plate.

RELATED APPLICATION This application is a continuation-in-part ofcopending application Ser. No. 194,651, filed Nov. 1, 1971, nowDefensive Publication T903,017.

BACKGROUND OF THE INVENTION In the making of flexible printing plates,usually a sheet of uncured rubber is placed in overlying relation with aflat rigid mold of thermosetting plastics material and in which isformed a recess of the impression to be printed. The mold and rubbersheet are placed within a press which is heated while the rubber sheetis pressed into the mold to cure the rubber and form the printing plate.A sheet of double face adhesive tape is frequently attached to the backsurface of the molded rubber printing plate, and the tape carries aprotective covering which is removed when it is desired to attach theplate to a roll or cylinder of a printing press.

While the double face adhesive tape provides for quickly attaching arubber printing plate to the outer cylindrical surface of a printingcylinder, it is difiicult to adjust the position of the plate preciselyon the surface of the supporting cylinder. That is, the plate must bepulled or ice stripped from the cylinder each time it is desired toshift the position of the plate by a fraction of an inch.

Another method of attaching a flexible rubber printing plate to acylinder is by constructing the cylinder so that it has a magnetizedouter surface which attracts a magnetically attractable rubber sheetwhich is laminated to the back of the rubber printing plate. One suchmagnetic cylinder is manufactured and sold by the Dayco Corporation,Dayton, Ohio. This company also produces an uncured rubber sheet inwhich is dispersed ferrous metal particles. This sheet is placedadjacent the sheet of uncured rubber mold material before the latter.sheet is cured within a heated press so that the magneticallyattractable rubber sheet is laminated to the rubber mold sheet duringthe molding and curing operation.

It has been found that a number of problems are encountered in themanufacturing and using of magnetically attractable rubber printingplates. For example, air is frequently entrapped between the uncuredrubber mold sheet and the uncured magnetically attractable rubber sheetwhen the sheets are being laminated together during the moldingoperation. This entrapped air prevents a continuous bond or laminationof the sheets. There is also a problem of air being entrapped within therecessed irnpression of the mold. This entrapped air results in pocketsbeing formed within the printing surface of the rubber late.

p It is also difficult to mold the combined rubber sheets to obtain aprinting plate of precisely uniform thickness or caliper. When thelaminated molded rubber printing plate varies by a few thousandths inthickness, the paper web or other sheet being printed must be pressedagainst the printing surface of the rubber printing plate withsubstantial pressure so that the entire printing surface printseffectively. This pressure significantly reduces the useful printinglife of the rubber plate.

In some rubber molded plates where the printing surface has fine detail,it has been found necessary to grind the back surface of the plate afterit is mounted in an inverted manner on the roll of a grinding machine.This grinding operation must be performed slowly to minimize thespringback of the surface as a result of the resiliency of the rubbermaterial. As a result, the grinding operation significantly increasesthe cost of using a rubber printing plate.

The printing surface of a rubber printing plate is also distorted whenthe printing plate, which is molded in the flat, is mounted on aprinting cylinder. That is, the axially spaced edges of the plate or ofthe printing surface tend to project above the remaining portion of thesurface when the plate is curved to conform to the curvature of thecylinder. The greater the thickness 01' caliper of the plate and/or thesmaller the diameter of the printing cylinder, the more pronounced thisdeformation becomes. Thus with small diameter cylinders such as thosehaving a. three inch or four inch outer diameter, it is frequentlydesirable to grind the back surface of the plate to achieve a uniformcaliper.

SUMMARY OF THE INVENTION The present invention is directed to animproved flexible printing plate which is ideally suited for mounting ona magnetic cylinder of a printing press and which substantially reducesthe time required for mounting and positioning the plate on thecylinder. The printing plate of the invention is also simple andeconomical to produce and provides a printing surface which is preciselyconcentric with the outer surface of the cylinder. As a result, theplate does not have to be ground and is capable of producing fine detailwith the minimum of pressure between the printing surface of the plateand the paper web or other sheet being printed so that the printing lifeof the plate is significantly increased. The flexible printing plate ofthe invention also provides for substantial magnetic attraction so thatit is positively retained in position on a magnetic printing cylinder.

In accordance with one embodiment of the invention, a flexible printingplate is constructed by casting a volume of liquid plastisol to form aflat face layer which is heated and cured within an oven. A metal powderis uniformly mixed by weight with another volume of plastisol, and themixture is cast over the cooled face layer, and the combined layers arereheated in an oven to cure the base layer and fuse it to the facelayer. The multilayer thermoplastic mold sheet is cooled and then shavedto remove the scale from the back surface of the base layer and providethe mold sheet with a generally uniform thickness or caliper.

A porous mold matrix or mat is placed in contact with the face layer ofthe mold sheet, and the combined mat and mold sheet are heated toapproximately 400. The mat and mold sheet are then pressed togetherwithin a press having a resilient diaphragm which is expanded to engagethe base layer and to surround the edges of the mold sheet. Theimpression within the mat is molded into the face layer of the moldsheet to form a flexible printing plate which is allowed to cool duringthe pressing operation.

After the printing plate is removed from the diaphragm press, the baselayer is heated, and the combined porous mat and printing plate areplaced within a caliper mold which is pressed within the diaphragm pressto bring the mold printing plate to a precise uniform caliper. The edgesof the printing plate are then trimmed, after which the plate isreheated to approximately 200 F. until the plate relaxes and becomessomewhat pliable. The reheated plate is positioned on a drum having anouter diameter the same as the magnetic printing cylinder whicheventually receives the plate, and the plate is held in firm contactwith the drum by a resilient elastic sheet which engages the outersurface of the plate.

In accordance with another embodiment of the invention, the mold sheetand mat are bounded by magnetically attractable strips before the moldsheet is heated and pressed into the mat. The strips serve to retain themat and resulting printing plate on a magnetic base plate secured to thebed of a shaving machine and provide for shaving the base layer toproduce a precisely uniform printing plate.

Other features and advantages of the invention will be apparent from thefollowing description, the accompanying drawings and the appendedclaims.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a fragmentary perspectiveview of a magnetic printing cylinder on which is mounted a flexibleprinting plate constructed in accordance with the invention;

FIG. 2 is an axial end view of the magnetic cylinder and printing plateshown in FIG. 1;

FIG. 3 is an enlarged fragmentary view of the printing cylinder and anenlarged section of the printing plate as taken generally on the line 33of FIG. 2;

FIG. 4 is a perspective view illustrating the casting of the face layerof the printing plate shown in FIGS. 1-3;

FIG. 5 is a diagrammatic section through an oven and illustrating thecuring of the face layer;

FIG. 6 is a perspective view similar to FIG. 4 and illustrating thecasting of a base layer onto the cured face layer;

FIG. 7 is a view similar to FIG. 5 and illustrating the curing of thebase layer and fusing of both layers;

FIG. 8 is a diagrammatic view of a shaving machine and illustrating theshaving of the outer surface base layer of the mold sheet formed in FIG.7;

FIG. 9 is a view similar to FIGS. 5 and 7 and illustrating the heatingof the mold sheet in contact with a porous mold mat;

FIG. 10 is a schematic section through a diaphragm press andillustrating the molding of the mold sheet to receive the impression ofthe porous mold mat;

FIG. 11 is a diagrammatic section of a heating unit and illustrating theheating of the base layer of the printing plate molded in FIG. 10;

FIG. 12 is a diagrammatic section through a calibration mold andillustrating the pressing of the printing plate to a precise caliper;

FIG. 13 is a diagrammatic view similar to FIG. 5 and illustrating thereheating of the printing plate after it is molded to a uniform caliper;

FIG. 14 is a radial section of a device for reforming the printing plateafter it is reheated as illustrated in FIG. 13;

FIG. 15 is a view similar to FIG. 3 and showing a modified form of aprinting plate constructed in accordance with the invention;

FIG. 16 is an enlarged fragmentary ection of a mold sheet and matassembly prepared in accordance with a modification of the invention andillustrated during the step of heating in the oven shown in FIG. 9;

FIG. 17 is an enlarged section similar to FIG. 16 and illustrating thepressing of the mold sheet and mat within the diaphragm press shown inFIG. 10;

FIG. 18 is an enlarged fragmentary section of the molded printing plateand mat assembly and schematically illustrating the shaving of the platewith the mat attached;

FIG. 19 is an enlarged section similar to FIG. 18 and illustrating theshaving of the printing plate after the mat is removed; and

FIG. 20 is an enlarged fragmentary section of a final printing plateafter it has been shaved and the border portion has been trimmed.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIGS. 1-3, amagnetic printing roll or cylinder 20 includes a series of axiallyarranged discs 22 which are mounted on a support shaft 23 and includecorresponding permanent magnets. The outer surface 24 of the cylinder 20is ground to be precisely concentric with the axis of the cylinder.

In accordance with the present invention, a flexibleprinting plate 25 ismounted on the outer surface 24 of the cylinder 20 and is positivelyretained by the magnetic attraction between the printing plate and themagnetic discs 22. However, while the flexible printing plate 25 isparticularly adapted for mounting on a magnetic printing cylinder, it isto be understood that the printing plate 25 may be attached to anon-magnetic printing cylinder with the use of double face adhesivetape.

The flexible printing plate 25 includes a face layer 28 which has aresilient raised detailed printing surface 29, as will be explainedlater. The face layer 28 is fused to a base layer 30 containing aferrous metal powder or particles 31 which are uniformly dispersed toprovide a substantial magnetic attraction with the outer surface 24 ofthe printing cylinder 20. As illustrated in FIG. 2, the outer printingsurface 29 of the plate 25 defines a cylindrical plane which isprecisely concentric with the outer surface 24 of the cylinder 20 sothat a minimum pressure is required between the outer surface 29 of theprinting plate 25 and the web of paper or other material which is toreceive the impression of the printing surface 29.

As indicated above, FIGS. 4-14 illustrate a method for producing theflexible thermoplastic printing plate 25. While the method steps haveproduced a printing plate 25 which prints with highly satisfactoryresults, it is to be understood that the invention is not limited tothese precise steps and that the steps may be modified or performed in adilferent manner without departing from the scope of the invention.However, the steps are illustrative of a simple and inexpensive methodfor producing a flexible printing plate which not only has a preciseuniform thickness or caliper C (FIG. 3), but is capable of being firmlyretained on a magnetic printing cylinder, and provides for fine detailedprinting over a long period of service.

Referring to FIG. 4, the face layer 28 of the printing plate 25 isinitially made by pouring or casting a liquid vinyl plastisol 35 from acontainer 36 onto the flat bottom wall 37 of a tray-like mold 38 whichincludes a frame 39 surrounding the bottom wall 37. One plastisolmaterial which has provided successful results is one produced by theOhio Sealer Company, Dayton, Ohio and sold under the number 7471C. Theplastisol 35 is poured to a substantially uniform depth of about .040"

' and is stirred gently with a probe to remove all air bubbles. The mold38 and the face layer 28 of plastisol 35 are placed within an oven 45having a rack 46 for supporting the mold 38 and electrical heatingelements 48 for maintaining the oven at a temperature of between 350 F.and 400 F. After the aluminum mold 38 and face layer 28 of plastisolhave been heated in the oven for approximately to 25 minutes to causethe resin of the plastisol to dissolve within the plasticizer andsolidify, the mold 38 is removed, and the face layer 28 is allowed tocool to substantially room temperature.

The base layer 30 is then poured or cast from a con tainer 52 onto thecool face layer 28 as illustrated in FIG. 6. The base layer 30 consistsof a mixture 55 of dry ferrous or iron powder within a volume of liquidvinyl plastisol. This mixture 55 is prepared in the container 52 bymixing the iron powder and the plastisol at a relative weight ratio ofapproximately 3.5 ounces of metal powder for each ounce of liquidplastisol. The iron powder and plastisol material are mixed slowly untilthe powder is uniformly dispersed through the plastisol and withoutforming any air bubbles in the mixture. The mixture 55 is poured intothe mold 38 to a substantially uniform depth of between .100" and .300"depending on the desired thickness for the base layer 30. One form ofiron powder which has provided successful results is one produced by thePyron Company, Niagara Falls, N.Y., and sold under lot number 3-BC.

The mold 38 with the cured face layer 28 and the liquid base layer 30,are returned to the oven 45 which, as mentioned above, is maintainedbetween 350 F. and

400 F. In approximately 15 to 25 minutes within the oven 45, theplastisol mixture cures and solidifies to form the base layer 30 and tofuse it with the face layer 28 to form a two-layer thermoplasticflexible mold sheet 60. The mold sheet 60 is allowed to cool toapproximately room temperature within the mold 38 after the mold isremoved from the oven 45.

After the cooled mold sheet 60 is removed from the mold 38, the sheet 60is placed with the face layer in contact with a horizontal movable flatbedplate 62 (FIG. 8) of a shaving machine having a shaving roll 64. Themold sheet 60 is fed under the shaving roll 64 to remove a scale whichis formed on the bottom surface of the base layer 30 when the layer wascured in the oven 45 shown in FIG. 7. Thus the shaving roll 64 exposesthe fine pores within the base layer 30 and provides the mold sheet 60with the generally uniform thickness or caliper.

Referring to FIG. 9, a porous matrix or mat 65 is formed from acompressed fibrous paper material and is molded with a recessedimpression 66 corresponding to the detail printing surface 29 desired onthe printing plate 25. The impression side of the mat 65 is positionedin contact with the face layer 28 of the thermoplastic mold sheet 60,and the other side of the mat 65 is placed in contact with a flat steelsupport plate 68. This assembly of the mold sheet 60, mat 65, andsupport plate 68 is placed back into the oven 45 and is heated toapproximately 400 F. The assembly is then removed from the oven 45, anda porous creeper cloth 70, consisting of a layer of nylon clothoverlying a layer of canvas duck material, is placed over the base layer30 of the mold sheet 60.

The heated mold sheet 60 and mat 65 are then placed wlthin a press 75with the support plate 68 contacting a lower press platen 76. The press75 includes an upper platen 78 which supports a resilient flexiblerubber diaphragm 82v secured to the platen 78 by a rectangular frame 83.Compressed air at a pressure of between 100 and 125 p.s.i. is suppliedthrough a passage 84 Within the upper platen 78 to the space behind thediaphragm 82 and causes the diaphragm to press the heated face layer 28of mold sheet 60 into the mat 65. The creeper cloth 70 is eflYective toprevent the mold sheet 60 from adhering to the diaphragm 82 and toprovide venting of air grzrtrapped between the mold sheet 25 and thediaphragm As illustrated in FIG. 10, the diaphragm 82 also surrounds theedges of the mold sheet 60 to resist lateral How of the mold sheet andthereby assure that the face layer 28 of the mold sheet fills the entireimpression 66 within the mat 65. As a result of the porosity of the mat65, the air which is trapped within the impression 66 of the mat 65during the pressing operation, is forced into and through the mat 65 forescape to atmosphere. This assures that the entire impression 66 withinthe mat 65 is filled with the heated plastisol forming the face layer 28and that no air pockets are formed within the resulting printing surface29 of the printing plate. While the face layer 28 is being pressed intothe impression 66 within the porous mat 65 under the pressure exerted bythe flexible diaphragm 82, the mold sheet 60 cools to form the printingplate 25.

Referring to FIG. 11, the assembly of the printing plate 25 and the mat65 are positioned on a support plate 88 so that the base layer 30 of theplate faces a heating unit 90 having a heating element 92 and analuminum grid 93 which uniformly diffuses the heat from the heatingelement 92. The base layer 30 is heated until it begins to soften on itsupper or back surface.

The assembly is then placed within a calibration mold 95 (FIG. 12) whichincludes a base steel plate 96 on which is mounted a caliper frame 98having a height corresponding to the desired calibration C for theprinting plate 25 and the thickness of the mat 65. The mat 65 is placedin contact with the base plate 96 of the calibration mold 95, and athick aluminum plate 102 is placed on top of the heated base layer 30 ofthe printing plate 25.

The assembled calibration mold 95 is placed within the press 75, and thediaphragm 82 is pressurized with air at a pressure of between 100 andp.s.i. This uniform pressure on the aluminum plate 102 causes the heatedbase layer 30 to expand or flow laterally, as illustrated by theprojecting edge portions of the layer 30 in FIG. 12, until the combinedmat 65 and printing plate 25 have a precisely uniform thicknesscorresponding to the height of the caliper frame 98. As the aluminumplate 102 conducts heat from the base layer 30 during the pressingoperation, the base layer cools. After the base layer has cooled tobelow 100 F., the press 75 is released, and the printing plate 25 andmat 65 are removed from the caliper 7 mold 95. The plate 25 is thenstripped from the mat 96, and the edges of the plate 25 are trimmed on ashear.

Referring to FIG. 13, after the printing plate 25 is stripped from theporous mat 65 and trimmed, the plate 25 is placed back within the oven45 and positioned on a curved support member 105. The plate 25 isreheated to a temperature of approximately 200 F. so that the platerelaxes and droops as indicated by the dotted lines in FIG. 13. Thereheated plate 25 is removed from the oven 45 and placed on a hollowcylindrical drum 108 which has an outer diameter the same as theprinting cylinder which is to receive plate 25. The reheated plate ispositioned with the base layer 30 in contact with the outer surface ofthe drum 108, and the plate is held in firm contact with the drum 108 bya press member 110 having a flexible rubber diaphragm 112 which ispressurized to engage the outer surface of the face layer 28. Thereheated plate may also be held in firm contact with the drum 108 by awide flexible rubber band which is stretched over the outer surface ofthe plate 25.

After the printing plate cools under the uniform pressure exerted by thediaphragm 112, the air pressure is released, and the plate 25 is removedfrom the drum 108. While the printing plate 25 may be stored in a fiatcondition, the reforming process of FIGS. 13 and 14 assures that theouter printing surface 29 of the plate 25 is precisely concentric withthe outer cylindrical surface 24 of the cylinder 20 when the plate ispositioned on the cylinder. The change in the printing surface 29 as aresult of the reforming operation is very minute and is usually notvisually detectable.

In reference to FIG. 15 which shows a modification of the invention, aprinting plate 125 is constructed in substantially the same manner asthe printing plate 25 except that the face layer 28 and base layer 30are formed as one layer 126, and the ferrous metal powder or particles31 are dispersed throughout the entire layer 126. That is, the layer 126is formed of the mixture 55 of liquid vinyl plastisol and ferrous metalpowder mixed at the weight ratio of approximately 1:3.5 as mentionedabove. Since the metal powder is exposed on the printing surface 129 ofthe plate 125, the printing surface 129 is harder and less resilientthan the printing surface 29 of the multiple layer printing plate 25.For many printing operations, however, the softer and more resilientprinting surface 29 of the plate '25 provides a slightly better printthan does the harder printing surface 129 of the plate 125.

Referring to FIGS. 16-20 which show a modified form of making a flexibleprinting plate in accordance with the invention, the mold sheet 60a ispositioned on a porous fibrous mat 650 which includes a rectangularframe-like border portion or flange 67a recessed to a levelcorresponding with the level of the recessed impression 66a. A series ofelongated rectangular bearers or retaining strips 132 consisting of thethermoplastic mixture 55, are positioned on the support plate 88 in aframe-like manner bordering or surrounding the corresponding edges ofthe rectangular mat 65a and the mold sheet 6011. The assembly of the mat65a, mold sheet 60a and retaining strips 132 are placed within the oven45 (FIG. 9) and are heated to approximately 350 F. for a period ofapproximately four minutes.

The heated assembly is then transferred to the press 75 where thediaphragm 82 is pressurized to a pressure of approximately 120 p.s.i. toeffect a uniform pressure of the mold sheet 60a against the mat 65a. Theheating and pressing operations are effective to form the impression 66awithin the mold sheet 60a as mentioned above, and also to fuse theframe-like arrangement of thermoplastic retaining strips 132 to thecorresponding outer edges of the thermoplastic mold sheet 60a. As alsomentioned above, the mold sheet 600 is allowed to cool while thepressure is being applied by the diaphragm 82 of the press 75, andhereby form the printing plate 25a.

After the assembly of the printing plate 250, the mat 65a and theretaining members or strips 132 are cooled and removed from thediaphragm press 75, the assembly is placed upon the upper surface 134 ofa magnetic base member or retaining plate 135 which is secured to thereciprocal bed 62 of a shaving machine having a shaving roll 64. Theretaining plate 135 includes a rectangular arrangement of closely spacedpermanent magnets 138 which are recessed within a plate-like body 139 ofan epoxy material. After the base plate 135 is mounted on the bed 62 ofthe shaver, the upper surface 134, of the base plate 135 is shaved bythe shaving roll 64 so that the surface 134 is precisely parallel withthe outer surface of the shaving roll 64 and remains precisely uniformduring linear movement of the bed 62.

When the assembly of the printing plate 25a, the porous mat 65a and theretaining strips 132 are placed or positioned on the magnetic base plate135, the retaining strips 132 are positively attracted to the magneticbase plate 135 as a result of the ferrous or iron particles dispersedwithin the thermoplastics mixture 55 from which the strips 132 aremolded or formed. Thus the printing plate 25a is positively retained orsecured to the bed 62 of the shaving machine while several thousandthsof material are shaved from the base layer 30a of the printing plate 25aby the rotating shaving roll 64.

As mentioned above, it is desirable for the printing plate 25a to have aprecisely uniform thickness or caliper which preferably does not varymore than .0005 inch. However, the thickness or caliper of the porousfibrous mat 65a cannot practically be held to such close tolerances andfrequently varies plus or minus .002 inch in thickness or caliper. Thusafter the base layer 30a of the printing plate 25a is shaved (FIG. 18)by one or more passes of of the shaving roll 64, the printing plate 25ais stripped or removed from the mat 65a, and the magnetic bearers orretaining strips 132 are sheared or trimmed from the printing plate 25a.The printing plate 25a is then placed or positioned back on the magneticbase plate 135 (FIG. 19) where the printing plate 25a is retained on thebase plate 135 by the magnetic attraction between the magnets 138 andthe ferrous particles within the base layer 30a of the plate 25a.

The printing plate 25a is again shaved to remove one or two thousandthsof material from the base layer of 30a and thereby provide the printingplate with a precisely uniform thickness or caliper at the printingsurface 29a. As a result of the outwardly projecting frame-like flange67a on the mat 65a, the intial printing plate 25a is provided with anintegral frame-like peripheral border portion 142 which has the samethickness or caliper as the remaining portion of the printing plate atthe printing surface 29a. After the final shaving operation (FIG. 19),the frame-like border portion 142 is trimmed from the printing plate 25a(FIG. 20). However, during the shaving operation, the border portion 142assists in assuring that the printing plate 25a is shaved to a preciselyuniform caliper along the entire printing surface 29a. That is, theborder portion 142 assures that the edges of the printing do not curveor roll downwardly during the shaving operation and thereby prevent auniform contact pressure of the printing surface 29a of the printingplate on the surface 134 of the magnetic retaining base plate 135.

From the drawings and the above description, it is apparent that aflexible thermoplastic printing plate constructed in accordance with theinvention provides several desirable features and advantages. Forexample, the printing plate 25 or 25a has a precisely uniform thicknessor caliper C which does not vary more than .0005 inch. The printingplate 25 or 25a also has a printing surface 29 or 29a which is preciselyconcentric with the outer surface of the cylinder on which the plate ismounted. This precise uniform caliper and uniform concentricity providefor minimizing the pressure between the printing surface 29 or 29a andthe paper web or sheet being printed. As a result, the printing plate 25or 25a has a relatively long printing life.

The construction of the printing plate from a liquid thermoplasticplastisol also provides an important feature of the invention. That is,the liquid vinyl resin and plastici zer provide for conveniently addingand mixing a fine iron powder into the liquid so that when the mixtureis heated to cause the resin to dissolve in the plasticizer, the mixtureis converted to a solid layer with the powder uniformly dispersed withinthe layer. This dispersion of the iron powder effectively provides for asubstantial magnetic attraction between the printing plate and amagnetic cylinder 20. While the preferred weight ratio of iron powderand liquid plastisol is approximately 3.5 :1, the weight ratio should beat least 2:1 to provide a satisfactory magnetic attraction. The liquidplastisol is also convenient to handle and, as illustrated in thedrawings, requires relatively simple and inexpensive equipment forproducing a high quality flexible printing plate. As a result, with onlya relatively small capital investment for equipment, high qualityprinting plates may be produced for a printing press equipped withmagnetic cylinders.

The use of the porous matrix or mat 65 and 65a and the resilient rubberdiaphragm 82 within the press 75 also provides important advantages.That is, the porous mat 65 or 65a provides for releasing air entrappedwithin the recessed impression 66 or 66a during the molding operation sothat the heated plastisol completely fills all of the detail of theimpression. The resilient diaphragm 82 is not only effective to provideuniform pressure against the mold sheet 60 or 60a, but also restrainsthe edges of the mold sheet to minimize lateral flow of the mold sheetand thereby assure filling of the impression 66 or 66a. This assuranceis especially important when the printing surface includes fine detail.

The operations illustrated in FIGS. 11 and 12 are also imporant in thatthey provide the thermoplastic printing plate with a precisely uniformcaliper and assure that every printing plate placed within the calipermold 95 re sults with the same caliper. Furthermore, the molding of theprinting plate 25a with the magnetic retaining bearers or strips 132(FIGS. 16 and 17) and the shaving of the printing plate 25a on themagnetic base plate 135 (FIGS. 18 and 19) provide another means forobtaining a flexible printing plate of precision caliper.

In addition, the reforming of a thermoplastic printing plate by thesteps illustrated in FIGS. 13 and 14, is effective to assure a preciselyconcentric printing surface 29 or 29a when the printing plate is mountedon a magnetic cylinder 20. That is, the reforming operation provides forproducing a relaxed plate which naturally conforms precisely to thecurvature of the cylinder without creating internal stresses within theplate. This is especially important when the plate is to be used on acylinder having a relative small diameter such as a diameter of threeinches.

While the methods and forms of printing plates herein describedconstitute preferred embodiments of the invention, it is to beunderstood that the invention is not limited to these precise methodsand form of printing plates described, and that changes may be madetherein without departing from the scope and spirit of the invention asdefined in the appended claims.

The invention having thus been described, the following is claimed:

1. A method of producing an improved flexible printing plate adapted tobe mounted on a magnetic cylinder of a printing press, comprising thesteps of casting a liquid vinyl material into a face layer having agenerally uniform thickness, heating said face layer to fuse saidmaterial, mixing a ferrous metal powder within a liquid vinyl materialto form a mixture, casting said mixture to form a base layer havinggenerally uniform thickness, combining said face layer and said baselayer and heating the combined layers to fuse said base layer to saidface layer and to form a mold sheet, placing said face layer of saidmold sheet in contact with a porous mat having a substantially uniformthickness and a recessed impression, heating said mold sheet to softensaid face layer, pressing said mold sheet against said mat with uniformpressure applied to said base layer to mold a raised printing surfacecorresponding to said impression within said face layer and to form aprinting plate, positioning a magnetically attractable vinyl retainingstrip adjacent an edge of said mold sheet and the corresponding edge ofsaid mat prior to heating and pressing said mold sheet, said retainingstrip being fused to said mold sheet during said heating and pressingoperations, positioning the assembly of said printing plate and saidporous mat on a magnetically attractable base member, machining saidbase layer to remove a portion of said base layer including said metalpowder for producing a printing plate having precisely uniform caliperand a base layer with exposed metal powder, stripping said porous matfrom said printing plate and severing said retaining strip from saidplate.

2 A method as defined in claim 1 wherein said magnetically attractablebase member is formed by positioning a plurality of permanent magnetswithin a plate-like body of plastics material, and shaving the uppersurface of said body.

3. A method as defined in claim 1 wherein a set of said retainingmembers are positioned to form a frame around said mold sheet and saidmat, and each of said retaining members is fused to said mold sheetduring said heating and pressing operations, and each of said retainingmembers are severed from said printing plate.

4. A method as defined in claim 1 including the step of reheating saidprinting plate after said base layer is machined until said platerelaxes, placing the base layer of the reheated plate on a drum havingan outer diameter corresponding to the outer diameter of the magneticcylinder, and applying a predetermined pressure to said printing surfaceof said printing plate while said printing plate cools on said drum.

5. A method as defined in claim 1 wherein said ferrous metal powder ismixed within said liquid vinyl material to form a weight ratio of atleast 2:1, respectively.

6. A method as defined in claim 5 wherein said weight ratio isapproximately 3.5 :1.

7. A method of producing an improved flexible printing plate adapted tobe mounted on a magnetic cylinder of a printing press, comprising thesteps of casting a liquid vinyl material into a face layer having agenerally uniform thickness, heating said face layer to fuse saidmaterial, mixing a ferrous metal powder within a liquid vinyl materialto form a mixture, casting said mixture to form a base layer having agenerally uniform thickness, combining said face layer and said baselayer and heating the combined layers to fuse said base layer to saidface layer and to form a mold sheet, placing said face layer of saidmold sheet in contact with a porous mat having a substantially uniformthickness and a recessed impression, heating said mold sheet to softensaid face layer, covering said base layer and the peripheral edges ofsaid mold sheet with the flexible diaphragm, applying a uniform fluidpressure against said diaphragm for pressing said mold sheet againstsaid mat to mold a raised printing surface corresponding to saidimpression within s a i i face layer and to form a printing plate,positioning a magnetically attractable vinyl retaining strip adjacent anedge of said mold sheet and the corresponding edge of said mat prior toheating and pressing said mold sheet, said retaining strip being fusedto said mold sheet during said heating and pressing operationspositioning the assembly of said printing plate and said porous mat on aprinting plate.

UNITED ing precisely uniform saliper and a base layer with exposed metalpowder, stripping said porous mat from said printing plate and severingsaid retaining strip from said References Cited STATES PATENTS Droitcour264293 Lippincott 101-40l.1 Hunn 264-Dig. 60 Leguillon 101 3s2 MV 1Welch 101382 MV Willett 264220 Jernt 101401.1 Kelly 101382 MV Thomas101--401.1 Gray 101-4011 Libberton 10l-395 De Maria et a1 10l395 ROBERTF. WHITE, Primary Examiner W. E. HOAG, Assist ant Examiner US. Cl. X.R.

